Influence of Ocean Acidification on the Organic Complexation of Iron and Copper in Northwest European Shelf Seas; a Combined Observational and Model Study

Abstract

The pH of aqueous solutions is known to impact the chemical speciation of trace metals. In this study we conducted titrations of coastal seawaters with iron and copper at pH 7.91, 7.37, and 6.99 (expressed on the total pH scale). Changes in the concentration of iron and copper that complexed with the added ligands 1-nitroso-2-napthol and salicylaldoxime respectively were determined by adsorptive cathodic stripping voltammetry-competitive ligand equilibrium (AdCSV-CLE). Interpretation of the results, assuming complexation by a low concentration of discrete ligands, showed that conditional stability constants for iron complexes increased relative to inorganic iron complexation as pH decreased by approximately 1 log unit per pH unit, whilst those for copper did not change. No trend was observed for concentrations of iron and copper complexing ligands over the pH range examined. We also interpreted our titration data by describing chemical binding and polyelectrolytic effects using non-ideal competitive adsorption in Donnan-like gels (NICA-Donnan model) in a proof of concept study. The NICA-Donnan approach allows for the development of a set of model parameters that are independent of ionic strength and pH, and thus calculation of metal speciation can be undertaken at ambient sample pH or the pH of a future, more acidic ocean. There is currently a lack of basic NICA-Donnan parameters applicable to marine dissolved organic matter (DOM) so we assumed that the measured marine dissolved organic carbon could be characterized as terrestrial fulvic acids. Generic NICA-Donnan parameters were applied within the framework of the software program visual MINTEQ and the metal-added ligand concentrations [MeAL] calculated for the AdCSV-CLE conditions. For copper, calculated [MeAL] using the NICA-Donnan model for DOM were consistent with measured [MeAL], but for iron an inert fraction with kinetically inhibited dissolution was required in addition to the NICA-Donnan model in order to approximate the trends observed in measured [MeAL]. We calculated iron and copper speciation in Northwest European shelf water samples at ambient alkalinity and projected increased pCO 2 concentrations as a demonstration of the potential of the approach.

The Effect of Metal Concentration on the Parameters Derived from Complexometric Titrations of Trace Elements in Seawater—A Model Study

“…pH was set to 8.05 and temperature 20 • C. The solubility and equilibrium constants from Liu and Millero (1999) were used to calculate iron hydroxide concentrations. The NICA parameters derived for Fe 3+ for shelf sea waters in Avendaño et al (2016) and the generic NICA parameters for H + , Ca 2+ , and Mg 2+ described in Milne et al (2003) were used in the model assuming dissolved organic carbon (DOC, 64 µmol L −1 ) was present as FA. Although the Donnan model was not excluded from the calculations, the Donnan volume is very small at the ionic strength of seawater and thus no iron was calculated as adsorbed in the Donnan layer.…”

“…Although the Donnan model was not excluded from the calculations, the Donnan volume is very small at the ionic strength of seawater and thus no iron was calculated as adsorbed in the Donnan layer. The added ligand, 1-nitroso-2-naphthol (NN), was included in visual MINTEQ for titration simulations (Gledhill and van den Berg, 1994;Avendaño et al, 2016). Three experimental designs for titrations were tested, each incorporating a total of 15-18 data points per titration.…”

“…However it was recently pointed out in Avendaño et al (2016), that the interpretation of titration data can also be influenced by inert species that do not react with the added ligand on the timescale of the titration experiment. Such inert species could include e.g., aged oxyhydroxides or other colloidal minerals that exceed their solubility product in the sampled seawater and that take a long time (>24 h) to equilibrate with the added ligand (Town and Van Leeuwen, 2005;Fitzsimmons et al, 2017).…”

“…Inert DFe will not (by definition) be in equilibrium with the added ligand but in titration data such species will manifest as "strong" ligands. Furthermore, if this inert fraction is determined by solubility rather than complexation, then the concentration of the inert fraction will increase with increasing iron concentration (Avendaño et al, 2016). Theoretically, datasets where such behavior occurs can be identified by careful examination of titration curves.…”

“…Total organic carbon concentrations in the open ocean are of the order of 40-100 µmol C L −1 , of which the major portion (∼90%) is considered to be in the dissolved fraction (DOC, <0.7 µm). In coastal waters, total organic carbon concentrations are enhanced, especially within and close to estuaries (Hoikkala et al, 2014;Avendaño et al, 2016). Despite the challenges associated with the analysis of such a complex mix of components in a highly saline matrix, much progress has been made in recent years in characterizing organic matter.…”